1. Technical Field
The present invention pertains to improvements in methods and apparatus for powering an electrodeless lamp with reduced Radio Frequency Interference (RFI). This invention has particular, although not limited, utility in lamps of the types disclosed in U.S. Pat. Nos. 5,504,391 (Turner et al), 5,448,135 (Simpson ), 5,404,076 (Dolan et al), 4,894,592 (Ervin et al), 4,859,906 (Ury et al), and 4,359,668 (Ury); the disclosures of these patents are incorporated herein by reference.
2. Discussion of the Prior Art
Electrodeless lamps of the type with which the present invention is concerned include a light transmissive envelope containing a plasma-forming medium known as fill. A microwave or Radio Frequency (RF) energy source has its output energy coupled to the envelope via a waveguide to excite a plasma in the fill, resulting in the discharge of light from the envelope.
FIG. 1 schematically illustrates one of the many possible configurations for an electrodeless lamp of the type with which the present invention is concerned. A lamp module 20 includes a magnetron 22 or some other source of RF or microwave electromagnetic energy. Energy from the magnetron 22 is coupled to a waveguide 24 via a coupling antenna 28 and into a screen cavity 30, in which a bulb 32 is disposed. Bulb 32 includes a generally spherical discharge envelope 34. The bulb 32 has a high pressure fill material contained within its discharge envelope 34 such as, for example, the material described in the above-referenced Dolan et al patent. Bulb envelope 34 is made of quartz or some other suitably transparent material. The screen cavity 30 is made from a conductive mesh or screen material opaque to RF or microwave radiation but transparent to light radiation.
In operation, the waveguide 24 directs the electromagnetic energy generated by the magnetron 22 into the screen cavity 30, exciting the fill atoms of noble gas (e.g., xenon, argon, krypton, etc.) in bulb 32, which is initially at room temperature, to effect discharge of electrons. The discharged electrons collide with other fill atoms causing a further discharge of electrons, thereby increasing the total population of free electrons. The increased population of free electrons results in increased collisions and increased temperature, and other atoms of solid or liquid fill material, such as sulfur, mercury, etc., are vaporized and emit the desired light radiation.
For compact lamp configurations, a coaxial resonator 26 includes screen 30 as an outer conductor, a center conductor 36 which preferably is hollow to carry cooling air, bulb 32, and coupling loop 40 which is used to provide a high voltage, exciting the electrodeless lamp bulb at a high energy density. Power is coupled from the waveguide to the resonator by a coupling loop 40 in which a horizontal RF magnetic field enters notch 38 behind conductor 36, inducing current in the base of the center conductor 36.
Difficulties have been encountered with the operation of the magnetron in the compact lamp configuration and in other lamps with a compact, highly resonant RF circuit. These difficulties are encountered in spite of careful impedance matching in the design of the lamp RF circuit. Impedance matching, in this context, refers to matching the bulb's impedance at full operating temperature to that of waveguide 24 whereby the impedance at antenna 28 is equal to the characteristic impedance for an RF circuit specified by the magnetron's manufacturer. A mismatch in impedance causes reflected RF energy to propagate back from the area of the mismatch and produces a higher Voltage Standing Wave Ratio (VSWR) than for a matched RF circuit. Thus, VSWR is used as a measure of impedance matching. For purposes of nomenclature, the lamp RF circuit includes the coupling antenna 28, the waveguide 24, the resonator 26 including the screen cavity 30 containing bulb 32, the RF load to be excited. In evaluating prototypes for compact lamps, it has been observed that, for compact RF circuit configurations, the VSWR is within acceptable limits at the selected frequency of operation for the magnetron, according to specifications provided by the manufacturer of the magnetron. In spite of this, the magnetron is adversely affected by the short waveguide and resonator 26 and exhibits unexpected behavior. Instead of the single selected frequency, multiple, spurious frequencies are observed in the magnetron output spectrum. As a result, lamp performance is adversely affected in two ways; the lamp light flickers in an unacceptable manner and the spurious frequencies produced are outside the 2400 MHZ to 2500 MHZ ISM band allocated under governmental guidelines for RF spectrum management. Spurious signals above and below the allocated band have been observed simultaneously. The frequencies produced change with the length of the waveguide between the magnetron and the lamp bulb.
Considerable research has resulted in no guidance as to how this unexpected problem may be alleviated. There is no information from the manufacturer of the magnetron on the phenomenon of high reflections at frequencies differing significantly from the magnetron operating frequency, where there is a good impedance match at the selected operating frequency. In fact, the manufacturer of one magnetron denies that it is possible to create a condition of oscillation at widely separated frequencies for RF circuits with an adequately matched impedance at the selected frequency.
In normal RF circuit design practice, the manufacturer provides design guidelines based on RF circuits with a matched characteristic impedance at the selected frequency of operation for a given magnetron. A magnetron is specified as having a given range of frequencies of operation and, in theory, the designer simply chooses a magnetron for oscillation at the selected frequency.
In the present situation, it appears that even though there is a well-matched load at the selected operating frequency, the magnetron (together with the lamp RF circuit) may also oscillate at a second or third widely separated frequency, 30 MHZ to 100 MHZ to either side of the selected frequency, when a VSWR of 20:1 or greater is present at the widely separated frequency. Thus, using the methods of the prior art, it is not possible to prevent spurious Radio-Frequency Interference (RFI) in electrodeless lamps having the shorter and more compact waveguide configurations. Unless a way is found to prevent the RFI emissions, fabrication of the smaller and more desirable electrodeless lighting fixtures will not be possible.